Porcine reproductive and respiratory syndrome virus (PRRSV) is a positive-strand RNA virus that belongs to the family of arteriviruses together with equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV) and simian hemorrhagic fever virus (SHFV, 14). PRRSV causes reproductive failure in pregnant sows and respiratory problems in piglets (20). It causes huge economic losses in -pig populations world wide. EAV causes reproductive failure and abortions in mares, and leads to persistently infected stallions. Infections with LDV or SHFV are mainly of importance as infections of experimental animals in the laboratory.
Vaccination against these Arterivirus infections is often cumbersome. Killed vaccines, in general, are not effective enough for most purposes, and although live-attenuated Arterivirus vaccines are available, it has been shown that some of these are not safe and still spread. Furthermore, these vaccines can not be distinguished from wild type field virus.
The genome of PRRSV, as an example of an Arterivirus genome, is 15.1 kb in length and contains genes encoding the RNA dependent RNA polymerase (ORFIa and ORFlb) and genes encoding structural proteins (ORFs 2 to 7; (14), (11)). Other Arterivirus genomes are somewhat smaller, but share the same genomic build-up, in that all synthesize subgenomic messenger RNA encoding the structural proteins.
The ORFs 2, 3, and 4 encode glycoproteins designated GP2, GP3, and GP4, respectively. ORF5 encodes the major envelope glycoprotein, designated GP5, ORF6 encodes the membrane protein M, and ORF7 encodes the nueleocapsid protein N. An additional structural protein (GP2b) is encoded by a small OFR, ORF2b. The analysis of the genome sequence of PRRSV isolates from Europe and North America, and their reactivity with monoclonal antibodies has indicated that; isolates from these continents are genetically distinct and must have diverged from a common ancestor relatively long ago (15).